1. Field
The invention has to do with the manufacture of cellulosic insulation material and with apparatus employed for the purpose.
2. State of the Art
A widely used insulating material for homes and other buildings is commonly manufactured by grinding shredded waste paper, particularly newsprint, in suitable equipment such as a hammer mill while beating a fire retardant composition such as boric acid thereinto, usually in the presence of just enough moisture to cool the mill and to prevent the giving off of dust. For this purpose, approximately 2% by weight of water is normally added to the mill while operating and the product is dry on discharge from such equipment.
Either boric acid or a borate is the preferred fire retardant composition in the industry; however, other less expensive and more readily available fire retardants have been substituted in whole or in part for these preferred retardants. Aluminum sulfate and ammonium phosphates have been the principal substitutes. These substitutes, though effective as fire retardants, have imparted many undesirable properties to the insulation and have caused many consumer complaints.
In recent years there has been an extreme shortage of boric acid and borates. This has led to greater use of the substitute fire retardants and to severe repercussions in terms of consumer complaints. Accordingly, federal specifications for cellulose insulation were adopted on June 15, 1978.
Federal Specification HH-I-515D, which has also been adopted by most state and local authorities, very severely curtails and in most cases prevents use in cellulose insulation of hygroscopic fire retardant chemicals. Among the federal specifications relating to fire retardants are:
A. Settled Density PA1 B. Moisture Absorption PA1 C. Corrosion PA1 D. Fungal Resistance
Specifications A and B are related. An insulation which settles loses thermal insulation values. Moisture absorption increases its weight and causes it to settle. Since untreated paper undergoing the applicable test will increase in weight about 7 to 9%, the limit normally being 15%, the fire retardant cannot be hygroscopic.
Corrosion is a severe problem. Tests are made on steel, aluminum, and copper. Many corrosion problems can be alleviated by pH control, but copper, ammonium salts, and carbon dioxide in the air form a copper ammonium carbonate, which results in extremely severe corrosion.
Fungal resistance is a problem in humid areas of the United States. Moisture absorption encourages this, and, together with the positive nutrient factors in ammonium salts, excludes such fire retardants from consideration.
Attempts have been made in the past to reduce the quantity of boric acid or borate retardant required to impart desired fire retardancy to waste paper insulation. The usual approach has been to soak the paper particles in the fire retardant solution, to press out excess moisture, and to then dry the paper. However, this has not proven to be economical and is rarely if ever used commercially. The pressed material remains approximately 80% by weight of the water and requires drastic drying procedures. The very high costs of the required drying equipment and of operating it make the process uneconomical, but in view of the fact that paper insulation, when soaked with a solution of boric acid or a borate and then dried, will require considerably less of such fire retardant as will insulation with dry add-on, many manufacturers have sought a commercially usable wet treatment for cellulose insulation.
Gerber U.S. Pat. No. 3,259,104, filed in 1962 represents one such attempt. Gerber injects steam into shredded newsprint insulation shortly prior to its being passed into a bagging collector. He utilizes no drying other than the air stream in which the insulation is carried, and the insulation has possibly two to three seconds of moisture contact prior to bagging. From about 15 to 28% of a mixture of boric acid and borax is used as a fire retardant, together with some aluminum sulfate. Despite the fact that he uses steam, Gerber states that the add-on rates are adequate whether steam is used or not. This is equivalent to saying that the steam contributes nothing. In 1962, dry add-on rates were from 15 to 25%, so no saving was achieved by this process.
A somewhat different approach is used in Kuechler U.S. Pat. No. 3,666,544 filed in 1970. There, hygroscopicity of certain fire retardants is utilized. A urea-diammonium, phosphate-precipitated silica composition in powder form is applied to cotton batting immediately prior to exposure of the latter to a humidified atmosphere in the presence of steam. The chemical mixture is extremely hygroscopic and soluble. Moisture condensate is absorbed by and dissolves this hygroscopic fire retardant and carries it into the pores of the cotton fibers. Laboratory tests performed by me show that the product remains wet. A 10% by weight addition of the fire retardant chemicals produces a 35% by weight moisture gain in the finished insulation.
Despite this prior art and the continuing need, no commercial wet process had been developed for the treatment of cellulose insulation with boric acid and/or borates up to the time of the present invention.